MBE Growth And STM Characterization Of 2D TI 1T'-WTe₂ And Stanene

Two-dimensional topological insulators(2D TIs)feature conducting edge states topologically protected from backscattering by time reversal symmetry and a gapped bulk band structure,thus hosting the quantum spin Hall(QSH)effect.The topologically protected gapless edge state is Spin-momentum interlock,which makes it immune from the scattering of defects and non-magnetic impurities,so they could form a dissipationless conductive channel.Due to these novel properties,two-dimensional topological insulators have a wide range of applications in future low-power devices,spintronic devices and quantum computing.In 2014,theoretical researchers predicted that two-dimensional transition metal dichalcogenides is a new class of two-dimensional topological insulator materials,including 6 kinds of materials such as 1T-WSe2 and 1T'-WTe2.These new topological materials have a stable structure and a large bulk gap,and are expected to realize the room-temperature quantum spin hall effect.Recently,other researchers predicted that the single-layer stanene which has a low-buckling structure is a two-dimensional topological insulator with a band gap as large as O.1eV.In addition,it also has many attractive properties.In this paper,we explore the growth of these two new two-dimensional topological insulator materials by molecular beam epitaxy technology,and study their electronic structure and topological properties by scanning tunneling microscopy and scanning tunneling spectroscopy.Here are the main experimental resultswhat we obtained:1.By using molecular beam epitaxy,we have successfully grown the high quality monolayer 1T'-WTe2 on graphene substrates.With the help of scanning tunneling microscope and scanning tunneling spectrum,we not only studied its crystal structure,but also directly observed the topological edge states,which confirmed the predicted results of the theory.2.We have characterized the single-layer 1T'-WTe2 with high-resolution QPI-STS/STM,and verified a semimetal band structure,where there exists a band inversion near the ? point,without a full SOC-induced bulk band gap.An important finding is the Coulomb gap induced by the electron interactions in this 2D localized electron system.The observation of the Coulomb gap in the single-layer QSH systems is vital to distinguish the topological edge states from the vanishing bulk band state and greatly facilitates the realization of QSH effect.3.By controlling the growth temperature and annealing time,we not only obtained high quality Stanene Ixl on Bi(111)substrates,but also obtaied a new Stanene R3.The crystal structures of the two phases were studied by scanning tunneling microscopy(STM).By scanning the tunneling spectrum,we find that the electron structures of the two phases are quite different.Stanene R3 has a flat shaped energy gap at the Fermi level.In addition,we also directly observed the edge states on R3 Stanene island,which indicate that Stanene R3 is likely to be a new two-dimensional topological insulator.